I have used an offcut of wood from the money boxes I made for this W.I.P.
First thing is to mark the centre line for the hole (I used a pen to highlight what I have done) This is the inside of the box
Take a small drill bit and drill a hole all the way through so as to give a reference on both sides of the piece
Take a forstner bit just over the size of the largest coin you wish to use and insert it into a drill press. Now from the inside bring the bit down to just mark the wood. as picture below
Now using a 1/2" Forstner bit drill 2 holes down to a depth of at least 1/4 of the thickness of the wood using the intersection of the outer mark you made with the first bit and the centre line you drew.
Reinsert the first fortner bit and drill down to the same depth and a 1/16" more.
Replace the forstner bit with one at least 1/2" larger than the one you have just removed. Now drill from the other side (outside) until you reach the hole you had just drilled
You may find that a little cleaning up with a sharp knife and sandpaper .
Please remember that little fingers will get pocked into the hole to retreave the money so a good sanding to get rid of sharp bits ( I have not cleaned up in the photos below as this is not going to be used)
As for the bung itself I simply turned this on the lathe from the same material. You could use hole saws, the cross/locking bar I made from a piece of thin modelling ply just glued in place the slot was mounted into a spare hole piece and held in place with double sided tape and simply routed the slot(this was done before fitting the cross/locking bar.)
You will need to make sure that it is a reasonably a loose fit so as to turn freely
Hope you all understand this and that it is of some use